The present invention relates generally to systems and methods for providing steering torque assistance to an electrically-powered steering apparatus of a vehicle and more specifically to a system and method for providing steering torque assistance to an electrically-powered steering apparatus of a vehicle in which torque assistance is provided in a continuous manner despite the implementation of torque-clipping.
In a conventional electric steering system, a vehicle operator may experience a requirement for an approximately constant level of operator-supplied hand-wheel torque throughout a full range of hand-wheel rotation until the hand-wheel, and/or the vehicle steering apparatus, reaches a physical limit. From the perspective of the vehicle operator, as long as no physical limitation is active, the hand-wheel responds to the operator's application of torque by traveling until the limit is encountered. Once the limit is reached, further application of torque by the operator does not result in further rotation of the steering wheel.
In an electrically assisted power steering system, it can be important to protect system components from overheating or other potentially detrimental conditions. In some systems, it has been observed that accumulated effects of various steering maneuvers may cause overheating or other issues if preventative measures are not implemented. One form of preventative measure involves over-riding commanded levels of steering assist so as to reduce loads (e.g., thermal loads, forces, fatigue, etc.) imposed on critical system components. When commanded levels of torque for assisting steering are over-ridden, the torque assistance is said to be de-rated. When torque assistance is de-rated, increased torque is required from the operator to rotate the hand-wheel to perform a particular maneuver with the vehicle.
Unfortunately, in some cases, the implementation of such protective, torque-limiting measures (i.e., de-rating) may tend to reduce system outputs in ways that negatively impact the operability or reliable feel of the steering system and thus the drivability of the vehicle. For example, one system and method imposes a maximum limit on torque assistance without otherwise affecting torque output so long as commanded levels of torque fall below the maximum limit.
In accordance with such torque-clipping systems and methods, as long as a driving maneuver does not require maximum assist from the torque assistance mechanism, a vehicle operator will not perceive any decrease in anticipated steering system performance. In effect, since the vehicle operator doesn't need maximum assist, the vehicle operator will receive all the steering assist the vehicle operator expects. However, as soon as the vehicle operator attempts to perform a steering maneuver that would normally call for a level of torque assistance that exceeds a de-rated, or clipped, maximum torque limit, the torque assistance provided by the system remains relatively fixed at the maximum allowed (i.e., clipped or de-rated) level.
From the perspective of an operator of a vehicle equipped with such a torque-clipping system, the level of torque input required from the vehicle operator in order to complete the maneuver suddenly increases in a perceptibly discontinuous manner. In such situations, even though the vehicle may have appeared to the operator to have been operating in a normal fashion, the vehicle may abruptly fail to respond as anticipated without the operator's input of increased hand-wheel torque. This characteristic can create the perception in the operator that the steering apparatus may have reached its travel limit when, in fact, it has not.
In addition, with conventional de-rating, the operator may experience a discontinuous relationship between hand-wheel input torque, resulting rotation of the hand-wheel, and turning rate of the vehicle. Put another way, conventional steering torque assistance systems may cause the operator to experience an ever-increasing level of hand-wheel torque required to rotate the hand-wheel (i.e., to turn the vehicle) at hand-wheel positions beyond the point at which a de-rated torque assistance limit is encountered, and such undesirable conditions may exist even though the steering apparatus has not reached an actual physical limit.
In some systems, clipping may be imposed only when one or more system criteria, such as thermal-related system criteria, are met. Such steering systems may produce a first set of response characteristics when the criteria are not satisfied (e.g., on cold days) and a completely different set of response characteristics when the criteria are met (e.g., on hot days, under extreme driving conditions). Still further, response characteristics of such protected systems (e.g., thermally-protected systems) may change abruptly from a non-torque-limited mode to a limited-torque mode and back to a non-limited mode within a single driving session. In such systems, the full steering torque assistance may be available until thermal limit clips the maximum output. When clipping occurs, however, there may be an abrupt, unanticipated loss of steering torque assistance, which the vehicle operator may perceive as an unexpected loss in responsiveness of the vehicle.
Accordingly, it is desirable to have a system and method for providing steering torque assistance, in which torque assistance is provided in a continuous manner despite the implementation of torque-clipping.